When reading data stored in a disk drive system, the stored data is sampled and then processed, for example using one or more error correction codes. The performance of a disk drive system (or any sampled data system for that matter) depends upon the ability to sample an input signal at or near the phase at which the system was designed to operate (i.e., an ideal or perfect sampling phase). A typical approach to ensuring such timing accuracy is to use decisions (an estimate of the transmitted/written signal) produced by a low latency detector to construct a timing error gradient. This gradient is then used as input to a feedback loop (e.g., a timing loop) which drives the sampling phase to its proper value.
This approach works when the quality of the decisions made by the low latency detector is sufficiently accurate (i.e., has a sufficient signal to noise ratio (SNR)). In poor conditions (e.g., low SNR), systems that use this approach fail. In a worst case scenario, the system loses lock where the sampling phase is completely unknown and the transmitted/written data becomes unrecoverable. The trend in disk drive systems is towards lower SNR, for example because of increased storage capacity as would be achievable with more powerful codes (e.g., low density parity check (LDPC) codes). In current disk drive systems it is not uncommon to observe phase offsets of ±15% (where a phase offset of ±50% corresponds to being 180° out of phase with respect to an ideal sampling phase); future systems may have phase offsets of ±25% or more. It would be desirable to develop new techniques and systems to handle lower SNR values. Furthermore, it would be desirable if at least some of these techniques and systems take into account considerations such as die size (which affects cost), power consumption, latency, etc.